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Cybernator
Registered: Jun 2002
Posts: 154 |
VIC features at 2MHz
I'm curious as to how 2MHz change things relating the VIC features/timing. As the DOT clock is exactly 8 times the phi-2 clock, two things can happen:
1. Increase the DOT clock to 2*2MHz (~16 MHz), so that the nr. of cycles per frame doesn't change, but we have faster screen refresh.
2. Leave the DOT clock at ~8 MHz, and have more CPU cycles per rasterline.
Now that I have access to a C128 (:-DDDD) I can see the first case is incorrect. (*)
Since on C64 I can _move_ the write access in 8-pixel steps, that means on C128 I can do that in 4-pixel steps. On the other hand, VIC's I/O accesses are synced to the phi-2 clock (which is now at 2 MHz), not the DOT clock. That would mean there're still 63 cycles per rasterline, i.e. the second case is wrong as well, but then the first case would have to be correct. ;P
Another possibility is to leave VIC at 1Mhz, while the CPU is at 2MHz. But then there would be access-conflict between VIC and CPU, I guess.
Hope you understand what I'm thinking of. ;)
Btw, the CPU clock on C64 is not exactly 1 MHz, but 0.985 MHz (IIRC). Thus I guess the CPU clock on C128 is not exactly 2 MHz. So what's the exact frequency then?
Also, are there any VDC features?
Would be cool to do something with this monster sometime. ;)
Btw2, how can I use CPM? Is there a disk image available for download, and can it work with a 1541? Is there anything cool I can do with Z80 that I can't do with 8502?
--
(*) Though I'm not exactly sure
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Oswald
Registered: Apr 2002
Posts: 5031 |
imagine a 1mhz square signal.
in 1mhz mode the cpu can use the bus in the upper phases of the square wave, and the vic accesses it in the lower phases (or vice versa).
if you turn 2mhz mode on cpu will acess the buss in both cases. this means vic can no more read (and display) what it wants, it will read what the cpu has (or leaves) on the bus.
You cannot change d021 each 4 pixel. as the vic's dot clock wont change in 2 mhz mode either, but that will be clearly explained by some vic guru instead of me.
usage for vic trix the 2mhz mode was seen in risen for oblivion where xbow turned 2mhz mode on, on the sideborders, so he could display more sprites.
cpm: you just need a cartridge or a disk for the real thing, but cant help where you find any of those.
z80: AFAIK it runs too slow to compensate the drawback of its many cycle intructions. (at 1mhz?) |
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Compyx
Registered: Jan 2005
Posts: 631 |
About the Z80:
In the C-128 it runs at 2 MHz, later versions of the Z-80 ran at speeds of up to 20MHz. The instruction set has some interesting instructions that can for example CMP and INC at the same time and it has 4-bit shift instructions (4x LSR/ASL/ROL/ROR in one instruction!).
Unfortunately I haven't done much coding with Z-80's so I can't really tell how it would compare to the 65xx85/xx-family of processors.
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CyberBrain
Administrator
Posts: 392 |
Sorry for going off-topic:
Can the 8502 and the Z80 run in parallel?
It could be pretty kewl to have 3 processors (incl. the diskdrive) doing some heavy math at the same time. |
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Oswald
Registered: Apr 2002
Posts: 5031 |
Quote: Sorry for going off-topic:
Can the 8502 and the Z80 run in parallel?
It could be pretty kewl to have 3 processors (incl. the diskdrive) doing some heavy math at the same time.
no. |
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T.M.R
Account closed
Registered: Dec 2001
Posts: 749 |
i think Steve Judd did a comparison, from memory a 3.5MHz Z80a only manages to be about twice as fast as a 1MHz 6502 when command times are taken into consideration so the clock down that the C128 does means it's going to be crawling. |
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TNT
Account closed
Registered: Oct 2004
Posts: 189 |
You could find some use for Z80 repeated memory copy/compare commands (LDDR/LDIR, CPDR/CPIR), all taking 21 T-states to copy/compare one byte, repeating max. 65536 times. |
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Graham
Account closed
Registered: Dec 2002
Posts: 990 |
The Z80 memory copy/fill commands are slower than normal Z80 memcopy/fill with normal opcodes which again is slower than 8502 LDA/STA. |
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Cybernator
Registered: Jun 2002
Posts: 154 |
Thanks for the replies so far.
So this 2MHz mode was not as cool as I imagined it. I'm curious as to how to switch between the CPUs, and how to access RAM above 64k? Also how can I do keyboard scan at lowlevel (talking about the additional keys).
Project64 seems to be poor on C128 stuff. I just downloaded "c128 System Guide.txt" which seems to be a large text file, but haven't looked at it yet.
Btw, another question comes to mind. How do I turn the MMU into 64 mode? (Since in 128 mode you can disable ROM images/shadows which VIC sees, and you can use different color memory banks)
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Ninja
Registered: Jan 2002
Posts: 407 |
Unlike VDC, the MMU is not available in C64-Mode.
And for the extended keys, check register $D02F (http://unusedino.de/ec64/technical/aay/c64/vic47.htm). Couldn't you imagine, that these extra-keys are accessible through the VIC? ;)) |
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Oswald
Registered: Apr 2002
Posts: 5031 |
also the test bit in d030 does wild things, you can get 60hz screen, and have the normal screen displayed in the top /bottom borders. Check risen from oblivion if you have a 128. |
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